Negative driveline torque control incorporating transmission state selection for a hybrid vehicle

ABSTRACT

A hybrid propulsion system for a passenger vehicle is provided. The system comprises a first and second electric motor in the drivetrain, in addition to a transmission. When each of the motors are providing braking torque, the transmission state is adjusted in response to the amounts of braking torque provided by the first and second motors.

BACKGROUND AND SUMMARY

Vehicles may be configured with a hybrid propulsion system that utilizesat least two different sources of torque for propelling the vehicle. Asone non-limiting example, a hybrid propulsion system for a vehicle maybe configured as a hybrid electric vehicle (HEV), wherein an electricmotor and an internal combustion engine may be selectively operated toprovide the requested propulsive effort. Similarly, during decelerationof the vehicle, the electric motor and engine can be selectivelyoperated to provide vehicle driveline braking in order to recapturekinetic energy of the vehicle. In this way, vehicle efficiency may beincreased.

As described by U.S. Pat. No. 6,890,283, the engine may be disconnectedfrom the drive wheels of the vehicle during deceleration by disengagingthe transmission. By reducing engine braking during vehicledeceleration, a greater amount of energy may be recaptured by the motor.

The inventors of the present disclosure have recognized a disadvantagewith the above approach. In particular, during some conditions, themotor may not be able to provide sufficient driveline braking torque.Therefore, engine braking can be provided in combination with the motorto provide the desired driveline braking torque. However, operating theengine to supplement driveline braking in addition to the motor mayreduce the level of energy recaptured during the braking event.

As another approach, the inventors herein have provided a hybridpropulsion system for a passenger vehicle, comprising at least one drivewheel; a first motor coupled to the drive wheel; a second motor; atransmission including a first end coupled to the drive wheel and asecond end coupled to the second motor; and a control system configuredto control operation of the first and second motor to each providevehicle braking torque, wherein transmission state is adjusted tocontrol an amount of braking torque provided by the engine in responseto an amount of braking torque provided by the first motor and an amountof braking torque provided by the second motor. In this way, a firstmotor and a second motor may be coordinated to provide sufficientvehicle braking, and the state of the transmission may be adjusted tofacilitate the distribution of braking torque between the motors andengine, thereby improving driver feel during deceleration.

As yet another approach described herein, the control system may beconfigured to vary a relative level of electrical energy generated bythe first motor and the second motor in response to an operatingcondition to generate vehicle braking. In this way, energy recaptureduring vehicle deceleration may be selectively performed by the firstmotor, second motor, or both motors while considering operatingconditions such as the state of the transmission operatively coupledbetween the motors, a condition of the energy storage device, and/or thespeed of the engine, among others.

It should be appreciated that the various concepts that have beenprovided in the Background and Summary are non-limiting examples andthat these and other approaches will be described in the DetailedDescription in greater detail. Additionally, the various approachesdescribed herein and the claimed subject matter are not necessarilylimited to addressing the above mentioned issues.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example hybrid vehicle propulsion system.

FIG. 2 is a flow chart depicting a high level control routine forachieving vehicle deceleration.

FIG. 3 is a flow chart depicting a control routine for controlling adriveline braking operation.

FIG. 4 is a graph illustrating maximum braking forces that may beachieved by the various components of an example hybrid propulsionsystem.

FIG. 5 is mode map representing the various braking modes that may beperformed by an example hybrid propulsion system.

FIG. 6 is a flow chart depicting a control routine for selecting adriveline braking mode.

FIG. 7 is a schematic illustration of the energy flow paths for a firstbraking mode of an example hybrid propulsion system.

FIG. 8 is a flow chart depicting a control routine for the first brakingmode.

FIG. 9 is a schematic illustration of the energy flow paths for a secondbraking mode of an example hybrid propulsion system.

FIG. 10 is a flow chart depicting a control routine for the secondbraking mode.

FIG. 11 is a schematic illustration of the energy flow paths for a thirdbraking mode of an example hybrid propulsion system.

FIG. 12 is a flow chart depicting a control routine for the thirdbraking mode.

FIG. 13 is a schematic illustration of the energy flow paths for afourth braking mode of an example hybrid propulsion system. (minor: needto remove energy flow arrow heads from summer blocks to CISG/ERAD sinceonly engine braking used and electric machines are shutdown)

FIG. 14 is a flow chart depicting a control routine for the fourthbraking mode.

FIG. 15 is a graph representing an example transmission shift strategy.

FIG. 16 is a flow chart depicting a control routine for controlling alevel of braking torque provided by a first motor and a second motorresponsive to thermal conditions and/or an efficient operating range ofthe motors.

DETAILED DESCRIPTION

A variety of control strategies are disclosed herein to enablecoordination of the hybrid propulsion system driveline during vehicledeceleration. In this way, synergies between the different propulsionsystem components may be achieved, thereby enabling increased fuelefficiency and improved drivability of the vehicle. While the presentapplication is described with reference to an HEV system utilizing anengine, two electric machines, and an automatic transmission, it shouldbe appreciated that the approaches described herein may be applied tosome other hybrid propulsion systems.

With a hybrid propulsion system such as the HEV system shown in FIG. 1,for example, an opportunity exists to coordinate torque ratio selectionfor the transmission including transmission gear and torque converterstate selection in response to the regenerative braking capability ofthe electric machines. Thus, an energy efficient approach fordecelerating the vehicle is possible while maintaining operation of theenergy storage device within its energy storage (e.g. state of charge(SOC)) and power exchange limits, thereby enabling further increase inenergy efficiency. By coordinating and controlling the powertrain, suchas the electric machine torque commands, gear selection, etc., therequested driveline braking force can be achieved while increasingenergy recovery.

As described herein, the term deceleration can refer to the varioustypes of vehicle deceleration that may be achieved by the hybridpropulsion system including vehicle braking and hill holding. Thepresent disclosure considers at least the following two types of vehicledeceleration requests. A first deceleration request may include arequest to control vehicle acceleration/deceleration without necessarilyrequiring user input, such as vehicle speed control on a down grade,hill decent control (HDC), and/or cruise control, for example. A seconddeceleration request may include a request to reduce vehicle speed toeither a lower speed or a complete stop based on an input received froma user, such as closed-pedal control, which is indicative of driverdemand, or coast down, and/or braking, for example.

These vehicle deceleration requests can be directly requested by theuser through the use of a brake system and/or via selection of drivelinecontrol. For example, the user can utilize a foot pedal or other userinput device to request increased deceleration or vehicle braking. Asanother example, a user can select or adjust a state of the transmissionby selecting a particular gear with regards to a manual transmission orby selecting a particular operating range with regards to an automatictransmission as another approach for increased vehicle deceleration.Furthermore, these requests can be controlled without necessarilyrequiring user input through the application of control strategies usedto achieve specific vehicle functionality such as adaptive cruisecontrol or hill decent control. In either case, the driveline of thehybrid vehicle can be used to provide the requested deceleration inorder to increase energy recovery and extend the life of the frictionbrakes.

With non-hybrid vehicle propulsion systems, the amount of engine brakingtorque, and hence vehicle deceleration, can be limited at a givenvehicle speed where a transmission downshift may not be permitted, forexample, due to engine over speed and/or transmission protection. Thus,the driver may resort to applying the friction brakes in these scenariosto achieve the requested vehicle deceleration, since engine brakingalone may not facilitate the requested deceleration of the vehicle.Furthermore, if a transmission downshift is initiated in order toincrease the level of engine braking, an upshift of the transmission mayshortly follow if there was excessive braking torque in the lower gear.As a result, transmission shift hunting may occur with sometransmissions due to the lack of a continuous range of authority in thepowertrain when regulating the driveline braking torque. Finally, abruptchanges in the driveline braking torque which occur when shifting may bedirectly perceived by the driver. The various vehicle braking approachesdescribed herein seek to address these issues by taking advantage of theregenerative capability of a hybrid powertrain in coordination withtransmission state selection, thereby providing enhanced negativedriveline torque control.

FIG. 1 illustrates an example hybrid propulsion system 100 for avehicle. In this particular example, hybrid propulsion system 100 isconfigured as an HEV, which may be operated in conjunction with a rearwheel drive (RWD) vehicle platform. However, the approaches describedherein may be applied to other vehicle platforms including front wheeldrive, four wheel drive, or all wheel drive systems. Hybrid propulsionsystem 100 includes a powertrain comprising an engine 110, a firstelectric motor 120, a transmission 130, and a second electric motor 140.

Engine 110 may include one or more combustion chambers or cylinders 112for combusting a fuel. As one non-limiting example, engine 110 may beoperated in what may be referred to as an Atkinson cycle. The engine maybe operated in an Atkinson cycle to achieve improved fuel efficiencyover similarly sized Otto cycle engines, whereby the electric motors maybe operated to assist the engine provide the requested driveline torque,for example, during acceleration of the vehicle. However, in otherexamples, engine 110 may be operated in an Otto cycle or other suitablecombustion cycle. The torque produced by engine 110 may be delivereddirectly to drive shaft 152. During some or all of the vehicle brakingmodes described herein, the engine may discontinue combustion of fuel insome or all of the cylinders. In this way, fuel efficiency may beincreased during vehicle braking.

As illustrated by FIG. 1, the first electric motor 120 and the secondelectric motor 140 may be arranged on opposite sides of transmission130. By varying an operating state of transmission 130, an amount oftorque transmitted between driveshafts 152 and 154 via transmission 130can be varied. Transmission 130 can include a torque convertercomprising an impeller 132 and a turbine 134. The transmission may beengaged or disengaged by varying a state of the torque converter to varythe torque transfer between impeller 132 and turbine 134. Furthermore,transmission 130 may include two or more selectable gear ratios that canbe used to vary the ratio of speed and/or torque that is exchangedbetween driveshafts 152 and 154. As one non-limiting example,transmission 130 may include six selectable gears, however, othertransmissions having more or less gears may be used. In alternativeembodiments, transmission 130 may be configured as a continuouslyvariable transmission (CVT) to enable the transmission to providecontinuously variable gear ratios to the driveline. Furthermore, inalternative embodiments, transmission 130 may be configured as adual-clutch (i.e. powershift) or automatically shifted manualtransmission both of which do not use a torque converter.

In some embodiments, motor 140 may be included as part of a motor system142. As one non-limiting example, motor system 142 may be configured aswhat may be referred to as an electric rear axle device (ERAD) system,however, other suitable configurations may be used. In FWD applications,motor system 142 could also connected to the final drive of a front axleat the output of the transmission, and would be referred to as anelectric front axle drive (EFAD) unit. ERAD system 142 can include anysuitable gearing to enable motor 140 to be operated independent of driveshaft 154. For example, as illustrated by FIG. 1, motor system 142 mayinclude a planetary gear set comprising a carrier (C), a sun gear (S),and a ring gear (R). By varying a state of the planetary gear set, anamount of torque exchanged between motor 140 and drive shaft 154 may bevaried. In this way, motor 140 selectively supply or absorb torque todrive shaft 154. In alternative embodiments, motor 140 may be coupleddirectly to drive shaft 154. Furthermore, in alternative embodiments,motor system 142 may be solely used to drive the rear wheels while theengine 110, first electric motor 120 and transmission 130 are solelyused to drive the front wheels. In this way, the transmission output isnot mechanically coupled to the motor system 142.

In some embodiments, motor 120 may be included as part of a motor system122. As one non-limiting example, motor system 122 may be configured aswhat may be referred to as a combined integrated starter/generator(CISG) system. In this particular embodiment, motor 120 is operativelycoupled with drive shaft 152 such that rotation of drive shaft 152results in a corresponding rotation of motor 120 and engine 110. CISGsystem 122 may be operated to assist starting of engine 110 and/or togenerate electrical energy that may be stored in an energy storagedevice 160. However, it should be appreciated that motor 120 may beconfigured in a motor system that includes any suitable gearing toenable motor 120 to be selectively operated independent from engine 110.For example a clutch may be used to provide an operative disconnect inbetween CISG system 122 and engine 110 to reduce frictional torquelosses from the engine while the CISG system is used to generateelectrical energy.

CISG system 122 and ERAD system 142 may be operated to exchange torquewith drive shafts 152 and 154, respectively. For example, CISG system122 can be operated to supply torque to drive shaft 152 in response toelectrical energy received from energy storage device 160. Similarly,ERAD system 142 can be operated to supply torque to drive shaft 154 inresponse to electrical energy received from energy storage device 160.In this manner, the CISG and/or ERAD can be operated to assist theengine to propel the vehicle.

Furthermore, CISG system 122 and/or ERAD system 142 can be selectivelyoperated to absorb torque from drive shafts 152 and 154, respectively,whereby the energy may be stored at energy storage device 160 orexchanged between CISG system 122 and ERAD system 142. For example,electrical energy generated by the ERAD can be supplied to the CISG torotate engine 110. Energy storage system 160 may include one or morebatteries, capacitors, or other suitable energy storage devices.

Drive shaft 154 may be operatively coupled to one or more wheels 170 viaa final drive unit 172. Each of wheels 170 can include a friction brake174 to provide supplemental braking for deceleration of the vehicle.

A control system 180 may be communicatively coupled to some or all ofthe various components of hybrid propulsions system 100. For example,control system 180 can receive operating condition information fromengine 110 such as engine speed, CISG system 122, transmission 130including the current gear selected, transmission turbine 134 and driveshaft 154 speeds, torque converter state, ERAD 142, energy storagedevice 160 including state of charge (SOC) and charge rate, wheels 170including vehicle speed, and the position of friction brakes 174.Control system 180 can receive a user input via a user input device. Forexample, control system 180 can receive a vehicle braking request from auser 190 via a pedal 192 as detected by pedal position sensor 194. Insome embodiments, control system can identify the angle of inclinationor grade of the road surface via an inclinometer or other suitabledevice.

Further, control system 180 can send control signals to engine 110 tocontrol fuel delivery amount and timing, spark timing, valve timing,throttle position, among other engine operating parameters, CISG system122 to control the amount of torque exchanged between motor 120 anddriveshaft 152, transmission 130 to change gear selection and to controlthe state of the torque converter, ERAD 142 to control the amount oftorque exchanged between motor 140 and driveshaft 154, energy storagedevice 160 to control the amount of energy received from or supplied tothe ERAD and CISG systems, and friction brakes 174 to vary an amount ofbraking force applied at the wheels 170 as will be described in greaterdetail herein. It will be appreciated by one of skill in the art inlight of the present disclosure that the control system can adjustoperating parameters of the various driveline components viaelectromechanical or electro-hydraulic actuators, or other suitabledevice.

Control system 180 can include one or more microcomputers, including amicroprocessor unit, input/output ports, an electronic storage mediumfor executable programs and calibration values configured as read onlymemory chip, random access memory, and/or keep alive memory, and a databus. Thus, it should be appreciated that control system 180 can executethe various control routines described herein in order to control theoperation of hybrid propulsion system 100.

To achieve optimum negative driveline torque control during a brakingoperation, the control system may be configured to increase and/ormaximize energy recovery while reducing and/or minimizing engine brakingby utilizing the regenerative braking capability of the electricmachines within the energy storage capacity and power exchangelimitations of the energy storage device.

FIG. 2 is a flow chart depicting an example high level control routinethat may be performed by the control system to achieve vehicle braking.At 210, the control system can operate one or more of the ERAD, CISG,and/or engine to perform driveline braking to cause vehicledeceleration. Driveline braking will be described in greater detail withreference to FIGS. 3-15. At 220, at least a portion of the energyreceived from the driveline braking operation can be converted andstored at the energy storage device. At 230, the friction brakes may becontrolled by the control system to supplement the driveline braking.Note that control system 180 may include an anti-lock braking system(ABS) or other traction control system that utilizes the friction brakes174 to supplement the driveline braking as will be described in greaterdetail herein.

FIG. 3 is a flow chart depicting an example control strategy forcontrolling driveline braking of hybrid propulsion system 100. At 310,the control system can assess operating conditions of the vehicle,including current, past, and/or predicted future operating conditions.As described herein, operating conditions may include, but are notlimited to, one or more of the following: energy level or state ofcharge (SOC) of the energy storage device, energy exchange rate with theenergy storage device, amount of torque exchanged between the drivelineand the ERAD or CISG, the position of a user input device such as brakepedal 192, ambient conditions such as air temperature and pressure,angle of inclination or grade of the road surface, transmission stateincluding selected gear and torque converter state, transmission turbineand output speeds, engine speed, vehicle speed, among other operatingstates of the engine, CISG, ERAD, transmission, and energy storagedevice.

At 312 it may be judged whether driveline braking is requested. Avehicle braking request may be initiated by a user and/or by the controlsystem. For example, where the user depresses a brake pedal or activatesan input device for requesting a braking operation for the vehicle, thecontrol system can receive the request from the user as shown in FIG. 1.As another example, the control system may request vehicle brakingresponsive to the operating conditions identified at 310 withoutreceiving a braking request from the user. In other words, the controlsystem may request vehicle braking, for example, during active cruisecontrol, HDC, or other traction control operations. Thus, it should beappreciated that the braking request may originate from the user or fromthe control system.

If the answer at 312 is no (i.e. no vehicle braking is requested), theroutine may return to 310 where the operating conditions may be againassessed until a braking request is initiated by the user or by thecontrol system. Alternatively, if the answer at 312 is yes (i.e. vehiclebraking is requested) the requested braking force may be identified at314. As one example, the amount of braking force requested may beidentified by the control system based on the operating conditionsassessed at 310 and/or the position or movement of the brake pedal bythe user.

At a given vehicle speed, a vehicle driveline braking (i.e. negativedriveline torque) request can be interpreted as a desired or a requestedbraking force for achieving a desired vehicle deceleration. The desiredvehicle deceleration and corresponding requested braking force may bedescribed by the following equation:

$\begin{matrix}{{{desired}\mspace{14mu}{vehicle}\mspace{14mu}{deceleration}} = d_{VEH}} \\{= \left. \frac{F_{B\_ DES} - F_{ROAD}}{\left( \frac{W_{veh}}{g} \right)}\Rightarrow F_{B\_ DES} \right.} \\{= {{d_{VEH}\left( \frac{W_{VEH}}{g} \right)} + F_{ROAD}}}\end{matrix}$

where F_(B) _(—) _(DES): desired driveling braking force request,F_(ROAD): road load,

$\frac{W_{VEH}}{g}\text{:}$vehicle mass

As one example, engine braking can be used to achieve the desireddriveline braking force via selection of the appropriate transmissionstate, including the gear and/or torque converter state, in order toregulate the amount of engine braking used to decelerate the vehicle.Thus, engine braking can be described by the following equation:

$F_{B\_ DES} = {F_{B\_ ENG} = {- \frac{T_{ENG} \cdot i_{TQ} \cdot i_{G} \cdot \eta_{G} \cdot i_{FD} \cdot \eta_{FD}}{R_{TIRE}}}}$

-   where, T_(ENG): engine braking torque, i_(TQ): torque multiplication    by converter, i_(G): gear ratio, i_(FD): final drive ratio η_(G):    transmission gearbox efficiency, η_(FD): final drive efficiency,    R_(TIRE): effective tire radius

By selecting the appropriate gear at a given vehicle speed, the amountof engine braking used to decelerate the vehicle can be increased ordecreased within the maximum and minimum engine speed constraints.However, a hybrid propulsion system such as system 100 may be operatedto apply one or more driveline braking (i.e. negative torque) sourcesincluding the engine, CISG, and ERAD to regulate the driveline brakingforce. Thus, the braking force applied by the engine, CISG, and the ERADcan be described by the following equations:

${F_{B\_ ENG} = {- \frac{{T_{ENG} \cdot i_{TQ} \cdot i_{G} \cdot \eta_{G}}\mspace{11mu}\ldots\mspace{11mu}{i_{FD} \cdot \eta_{FD}}}{R_{TIRE}}}},{F_{B\_ CISG} = {- \frac{{T_{CISG} \cdot i_{TQ} \cdot i_{G} \cdot \eta_{G}}\mspace{11mu}\ldots\mspace{11mu}{i_{FD} \cdot \eta_{FD}}}{R_{TIRE}}}},{F_{B\_ ERAD} = {- \frac{T_{M} \cdot i_{ERAD} \cdot i_{FD} \cdot \eta_{FD}}{R_{TIRE}}}}$

-   where, F_(B) _(—) _(ENG): braking force from engine, F_(B) _(—)    _(CISG): braking force from CISG, F_(B) _(—) _(ERAD): braking force    from ERAD T_(ENG): engine braking torque, T_(CISG): CISG generating    torque, T_(M): ERAD motor generating torque η_(G): transmission    gearbox efficiency, i_(G): transmission gear ratio, i_(TQ):    transmission torque converter torque ratio i_(ERAD): ERAD gear ratio    to motor, η_(FD): final drive efficiency, i_(FD): final drive ratio

The total driveline braking force available for decelerating the vehiclecan be described by the following equations as the summation of theabove negative torque sources:

$\begin{matrix}{F_{B\_ PT} = {F_{B\_ ENG} + F_{B\_ CISG} + F_{B\_ ERAD}}} \\{= \frac{\left\lfloor {{\left( {{- T_{ENG}} - T_{CISG}} \right) \cdot i_{TQ} \cdot i_{G} \cdot \eta_{G}} - {T_{M} \cdot i_{ERAD}}} \right\rfloor \cdot i_{FD} \cdot \eta_{FD}}{R_{TIRE}}}\end{matrix}$ or driveline  braking  torque:T_(B_PT) = [(−T_(ENG) − T_(CISG)) ⋅ i_(TQ) ⋅ i_(G) ⋅ η_(G) − T_(M) ⋅ i_(ERAD)] ⋅ i_(FD) ⋅ η_(FD)where $\begin{matrix}{{{the}\mspace{14mu}{vehicle}\mspace{14mu}{deceleration}} = d_{VEH}} \\{= \frac{F_{B\_ PT} - F_{ROAD}}{\left( \frac{W_{VEH}}{g} \right)}}\end{matrix}$

In this way, the requested braking force can be identified for a givenvehicle deceleration that may be requested by the user and/or thecontrol system of the vehicle.

At 316, a driveline braking mode may be selected based on the operatingconditions assessed at 310 and the requested braking force identified at314. For a given requested braking force (F_(B) _(—) _(DES)), oralternatively a requested negative driveline torque, the control systemmay apply a rule-based state machine scheme in order to increase and/ormaximize energy recovery while achieving the requested driveline brakingforce. A braking mode may be selected based on the magnitude ofdriveline braking force requested, current SOC conditions and/or powerexchange limitations of the energy storage device, among otherlimitations of the driveline components.

A braking mode may be selected by the control system from a plurality ofbraking modes shown in greater detail by FIGS. 7, 9, 11, and 13. As oneexample, the braking mode may be selected by the control system inresponse to stored values, which may be represented by a drivelinebraking mode shown in FIG. 5. The control system may also utilizeadaptive learning to select a suitable braking mode based on previousdriveline braking response.

Note that the level of braking force provided by the CISG and the ERADmay be constrained by their respective limitations. Additionally, theenergy storage capacity (e.g. battery state of charge (SOC)) and/orpower exchange rate limitations of the energy storage device may furtherlimit the level of braking force that may be provided by the CISG andERAD. As one example, the driveline braking capabilities of both theCISG and ERAD may decrease with increasing vehicle speed as depicted bythe graph of FIG. 4. Furthermore, the braking capabilities of the CISGand the engine may also vary as a function of the transmission state ata given vehicle speed since the engine speed will increase or decreasein response to transmission shifting.

FIG. 4 provides a graph depicting braking limitations of the ERAD, CISG,and engine with varying vehicle speeds and transmission states forhybrid propulsion system 100. In particular, an example of the maximumbraking force that may be provided by the ERAD with varying vehiclespeed is shown at 410. A range of maximum braking force that may beprovided by the CISG is shown at 420-430 based on the particulartransmission gear selected. For example, a maximum braking force thatmay be provided by the CISG when a first gear of the transmission isselected is shown at 420. Examples of the maximum braking force that maybe provided by the CISG when one of a second, third, fourth, fifth, andsixth gear of the transmission is selected are shown at 422, 424, 426,428, and 430, respectively. As depicted by the graph of FIG. 4, themaximum amount of braking force that may be provided by the CISGdecreases with increasing vehicle speeds and is greater at lower gearsthan at higher gears.

FIG. 4 also shows a range of braking forces 440-450 that may be providedby the engine with varying vehicle speeds. For example, a maximumbraking force that may be provided by the engine when a first gear ofthe transmission is selected is shown at 440. Examples of the maximumbraking force that may be provided by the engine when one of a second,third, fourth, fifth, and sixth gear of the transmission is selected areshown at 442, 444, 446, 448, and 450, respectively. As depicted by thegraph of FIG. 4, the amount of braking force that may be provided by theengine increases with increasing vehicle speeds and is greater at lowergears than at higher gears. Note that the selected transmission state(e.g. transmission gear and/or torque converter state) can take intoaccount engine speed limits (e.g. lug and/or overspeed) in addition tofriction element energy limitations at a given vehicle speed. Thus, thecapabilities of the CISG and engine to provide driveline braking may bealso constrained by the maximum and/or minimum allowable engine speeds.

FIG. 4 also depicts road load for varying road grades and changingvehicle speed. For example, a 0% grade is shown at 460, a −5% grade isshown at 462, a −10% grade is shown at 464, a −15% grade is shown at466, a −20% grade is shown at 468, and a −25% grade is shown at 470.Note that a negative grade as described herein refers to a vehicletraveling down an inclined surface.

Thus, FIG. 4 illustrates example limitations of the engine, CISG, andERAD of hybrid propulsion system 100 with varying vehicle speed andtransmission state. Note that these limitations have been provided as anexample and may vary with the particular driveline configuration anddriveline actuators utilized by the control system. As shown in FIG. 4,during some conditions two or more of the engine, CISG and ERAD may beoperated to provide the requested driveline braking force if the maximumbraking force of any one of the driveline components is exceeded. Notethat the friction brakes may also be used to reduce driveline braking inorder to avoid limitations of the various driveline components. Thus,particular combinations of the engine, CISG, and ERAD, which are definedherein by several different braking modes A-D, may be used to providethe requested braking force as will be described in greater detail withreference to FIGS. 6-14 with or without requiring operation of thefriction brakes. In this way, the limitations on the amount of drivelinebraking force provided by each of the engine, CISG, and ERAD may beconsidered when selecting a particular braking mode so that therequested driveline braking force is achieved.

FIG. 5 illustrates a mode map representing an example control strategythat may be performed by the control system for selecting a braking modein response to the particular operating conditions of the vehicle. Eachof the braking modes can be selected to provide vehicle drivelinebraking in response to the energy storage and energy exchangelimitations of the energy storage device, while also avoiding conditionswhere limitations of a particular driveline component are exceeded, forexample, as described with reference to FIG. 4.

The example mode map of FIG. 5 includes four different mode regionsrepresenting conditions where one of modes A-D may be performed.However, it should be appreciated that in some embodiments, the controlsystem may select from a group of braking modes having greater than orless than four modes. For example, the control system may be configuredto select between only two or three different braking modes. Thevertical axis of the mode map corresponds to the requested drivelinebraking force and the horizontal axis of the mode map corresponds to theamount of energy recovered from the braking operation and/or the stateof charge of the energy storage device (e.g. a battery).

The vehicle control system can utilize the mode map of FIG. 5 to selecta braking mode in response to the requested driveline braking force, thebattery SOC, and/or the rate of energy recovery. For example, Mode B maybe selected during conditions of a higher battery SOC, while Mode A maybe selected during conditions of a lower battery SOC. Note that in thisexample, the energy recovered by the hybrid propulsion system during thebraking operation where Mode A is performed can be greater than during abraking operation where Mode B is performed, as indicated by the modemap. As another example, Mode A may be selected during conditions of alower requested driveline braking force and a lower battery SOC, whileMode C may be selected during conditions of a higher requested drivelinebraking force and a lower battery SOC. Selection of the drivelinebraking mode will be described in greater detail with reference to FIG.6.

Returning to FIG. 3, at 318, the selected braking mode selected at 318may be performed as will be describe in greater detail with reference toFIGS. 7-15. At 320, changes to the operating conditions may beidentified by the control system. At 322, it may be judged whether thebraking mode selected at 316 should be updated based on the change ofoperating conditions identified at 320. For example, the control systemmay compare a change in the operating conditions to the mode map toidentify whether a different braking mode is to be performed. As onenon-limiting example, the amount of driveline braking force requestedmay have changed, for example, in response to a change of the user inputor in response to a change in the operating conditions assessed by thecontrol system. As another example, the driveline braking mode may beupdated in response to a thermal condition and/or an efficient operatingrange of one of the motors as described in FIG. 16.

If the answer at 322 is yes (i.e. the braking mode is to be updated),the braking mode may be updated in response to the change in operatingconditions, whereby the control system may transition the vehicle to anupdated braking mode at 324. For example, the control system may adjustone or more operating parameters of the vehicle to transition betweenone of modes A, B, C, or D to another one of modes A, B, D, or D. At326, the updated braking mode may be performed.

Alternatively, if the answer at 322 is no (i.e. the braking mode is notto be updated) or from 326, the control system may judge whether thebraking operation is to be discontinued. For example, the control systemmay judge that the braking operation is to be discontinued whendriveline braking is no longer requested either by the user or by thecontrol system. As one example, the control system may judge that thebraking operation performed at 318 or 326 is to be discontinued when theuser removes their foot from the brake pedal or discontinues theirbraking request. As another example, the control system may discontinuethe braking operation when the operating conditions are changed suchthat braking of the vehicle is no longer requested, such as in responseto a change in road surface grade.

If the answer at 328 is yes (i.e. the braking operation is to bediscontinued), the control system may discontinue the braking operationat 330. The routine may then return to 310 as described above.Alternatively, if the answer at 328 is no (i.e. the braking operation isto be continued), the routine may return to 322 where the braking modeis continued and where it may be judged whether the braking mode is tobe updated. In this way, the selected braking mode may be adjusted inresponse to changes in operating conditions.

As described above, the routine depicted by FIG. 3 may be used toachieve driveline braking of a hybrid vehicle by utilizing one or morebraking modes. In this way, two or more sources of braking torque may becoordinated while providing increased energy recovery and consideringthe various limitations of the driveline components.

FIG. 6 is a flow chart depicting an example control strategy forselecting a braking mode, for example, as performed at 316. At 610 itmay judged whether the requested or desired braking force (F_(B) _(—)_(DES)) is greater than the maximum braking force of the ERAD (F_(B)_(—) _(ERAD) _(—) _(MAX)). If the answer at 610 is no, it may be judgedat 612 whether the state of charge of the energy storage device(B_(SOC)) is greater than the maximum or a threshold state of charge ofthe energy storage device (B_(SOC) _(—) _(MAX)) or it may be judged at614 whether the power supplied to the energy storage device (P_(BAT)) isgreater than the energy exchange limitation of the energy storage device(P_(BAT) _(—) _(MAX)).

Note that in an alternative embodiment the control system may also takeinto account thermal limits of the CISG & ERAD in selecting a brakingmode. For example, where during a braking operation, the ERAD reaches orexceeds a thermal limit, the CISG may be operated to provide increasedbraking so that the braking provided by the ERAD may be reduced. Also,generating efficiencies of both the CISG & ERAD could also be consideredwhen selecting a braking mode. For example, the ERAD may be operated toprovide vehicle braking within an efficient operating range, whereby theCISG may be operated to provide supplemental braking that exceeds theefficient operating range of the ERAD.

If the answers at 612 and 614 are no, mode A may be performed at 616.During mode A operation, the ERAD may be controlled to provide therequested driveline braking force and electrical energy generated by theERAD may be stored at the energy storage device, for example, asdescribed with reference to FIGS. 7 and 8.

Alternatively, if the answer at 612 or 614 is yes, mode B may beperformed at 618. During mode B operation, the ERAD may be controlled toprovide the requested driveline braking force. A first portion of theelectrical energy generated by the ERAD may be stored at the energystorage device and a second portion of the electrical energy may besupplied to the CISG where it may be used to supply torque to driveshaft 152, thereby providing kinetic energy to engine 110. For example,the CISG may be operated to increase the rotational energy of theengine. By varying the relative ratio of the first portion of energysupplied to the energy storage device and the second portion of energysupplied to the CISG, the energy storage capacity and energy exchangelimitations of the energy storage device may be avoided. Mode B will bedescribed in greater detail with reference to FIGS. 9 and 10.

Alternatively, if the answer at 610 is yes (i.e. the requested brakingforce is greater than the maximum or threshold braking force that is tobe provided by the ERAD), the routine can proceed to 620 and 622. At620, it may be judged whether the state of charge of the energy storagedevice (B_(SOC)) is greater than the maximum or a threshold state ofcharge of the energy storage device (B_(SOC) _(—) _(MAX)) or it may bejudged at 622 whether the power supplied to the energy storage device(P_(BAT)) is greater than the energy exchange limitation of the energystorage device (P_(BAT) _(—) _(MAX)).

If the answer at 620 and 622 are no, mode C can be performed at 624.During mode C operation, torque can be transmitted from the wheelsthrough the transmission to the engine and CISG. In some examples, boththe CISG and ERAD can be controlled to absorb torque from the driveline,where it may be converted and stored at the energy storage device. Itshould be appreciated that mode C can be used to provide the greatestdriveline braking force since the engine, CISG, and ERAD may be operatedto provide driveline braking. Mode C will be described in greater detailwith reference to FIGS. 11 and 12.

Alternatively, if the answer at 620 or 622 is yes, mode D can beperformed. During mode D operation, substantially all of the drivelinebraking can be provided by the engine. By discontinuing drivelinebraking by the CISG and ERAD, driveline braking may be performed by theengine without producing electrical energy during the braking operation.For example, mode D can be performed where a threshold or maximum SOC ofthe energy storage device has be reached or exceeded. Mode D will bedescribed in greater detail with reference to FIGS. 13 and 14.

The various braking modes depicted in FIGS. 5 and 6 will be described ingreater detail with reference to FIGS. 7-14. In particular, FIGS. 7, 9,11, and 13 provide schematic illustrations of energy flows for hybridpropulsion system 100 for each of modes A-D. In the examples of FIGS.7-14, the CISG is described as electric motor 1 (E/M 1), the ERAD aselectric motor 2 (E/M 2), and the energy storage device is configured asa battery.

Mode A

FIG. 7 shows a schematic depiction of hybrid propulsion system 100performing braking mode A. As shown schematically by FIG. 7, brakingmode A includes the use of only the ERAD to achieve the requestedbraking force, whereby the ERAD converts the braking force into energythat may be stored by the energy storage device. Thus, the ERAD providesbattery charging during the mode A braking operation while the engineand CISG are decoupled from the drive wheels.

As described above with reference to FIGS. 5 and 6, if the currentbattery SOC is less than a threshold SOC, the battery power limitationsare less than a threshold, and the requested driveline braking force iswithin the capability of the ERAD, mode A may be performed. Mode A canbe advantageously performed during these conditions to improve energyrecovery over modes B, C, and D since kinetic energy received from thedrive wheels are not used to rotate the engine. When operating inbraking mode A, the transmission is disengaged and the torque absorbedby the ERAD can be controlled in order to achieve the requesteddriveline braking force within the battery power absorption limits. Thelevel of ERAD braking force can be described by the following equation:

$\begin{matrix}{F_{B\_ DES} = F_{B\_ ERAD}} \\{= \left. {- \frac{{T_{M\_ DES} \cdot i_{ERAD}}\mspace{11mu}\ldots\mspace{11mu}{i_{FD} \cdot \eta_{FD}}}{T_{TIRE}}}\Rightarrow T_{M\_ DES} \right.} \\{= {- \frac{F_{B\_ DES} \cdot R_{TIRE}}{i_{ERAD} \cdot i_{FD} \cdot \eta_{FD}}}}\end{matrix}$

-   -   where, F_(B) _(—) _(DES): desired driveline braking force, F_(B)        _(—) _(ERAD): braking force from ERAD T_(M) _(—) _(DES): desired        ERAD generating torque, i_(ERD): ERAD gear ratio to motor,        η_(FD): final drive efficiency, i_(FD): final drive ratio,        R_(TIRE): effective tire radius

FIG. 8 is a control routine that may be performed by the control systemto achieve driveline braking via mode A. At 810 it may be judged whetherdriveline braking is to be performed in mode A. If the answer at 810 isno, the routine may return. Alternatively, if the answer at 810 is yes,it may then be judged whether the transmission is engaged. As oneexample, it may be judged that the transmission is engaged when thetransmission transmits torque between drive shaft 152 and 154, forexample, where the transmission is in a gear other than neutral.

If the answer at 812 is yes (i.e. the transmission is engaged), thecontrol system can disengage the transmission at 814 by increasing theslip in the torque converter and by transitioning the transmission intoneutral. For example, the transmission can be fully disengaged in modeA. Alternatively, if the answer at 812 is no or from 814 where thetransmission is disengaged, the ERAD may be controlled at 816 to providethe requested driveline braking force (or torque). For example, thecontrol system may adjust an actuator of the ERAD to increase the torqueabsorbed from the driveline by the ERAD. At 818, the energy generated bythe ERAD in response to the amount driveline braking it provides may bestored at the battery or other energy storage device. Finally, theroutine may return.

Mode B

FIG. 9 shows the energy flows of hybrid propulsion system 100 whileperforming braking mode B. Braking mode B again includes the use of theERAD to provide the requested braking force, whereby the ERAD convertsthe energy generated by the braking force into a first portion and asecond portion. The first portion of the energy produced by the ERAD maybe stored by the energy storage device. A second portion of the energyproduced by the ERAD may be used to drive the CISG, which in turn canrotate the engine. Thus, the ERAD provides battery charging and powersthe CISG during the mode B braking operation. The transmission isdisengaged during mode B so that torque is not transmitted to the engineand CISG. This prevents any braking force from the engine and CISG to betransmitted to the wheels. In order to achieve the requested drivelinebraking force without violating the battery's SOC and power limits, theCISG can be used to dissipate any extra braking energy produced by theERAD by supplying torque to driveshaft 152, thereby increasing thekinetic energy of the engine. In other words, the ERAD generating torquecan be controlled to meet the requested driveline braking force and theCISG motoring torque can be controlled to regulate battery power and SOCby dissipating excess energy through the engine as directed by thefollowing equations

$\begin{matrix}{F_{B\_ DES} = F_{B\_ ERAD}} \\{= \left. {- \frac{T_{M\_ DES} \cdot i_{ERAD} \cdot i_{FD} \cdot \eta_{FD}}{R_{TIRE}}}\Rightarrow T_{M\_ DES} \right.} \\{= {- \frac{F_{B\_ DES} \cdot R_{TIRE}}{i_{ERAD} \cdot i_{FD} \cdot \eta_{FD}}}}\end{matrix}$ $\begin{matrix}{P_{BAT} = \left. {P_{CISG} + P_{M}}\Rightarrow T_{CISG\_ DES} \right.} \\{{= \frac{P_{BAT} - P_{M}}{\omega_{ENG} \cdot \eta_{CISG}}},{T_{ENG\_ DES} - T_{CISG\_ DES}},}\end{matrix}$

-   -   where T_(CISG) _(—) _(DES): desired CISG motoring torque,        P_(BAT): desired battery power (f(SOC)), P_(M): desired ERAD        power, P_(CISG): desired CISG power, ω_(ENG): engine speed,        η_(CISG): CISG efficiency

It should be appreciated that braking mode B will typically recover lessenergy during a similar braking operation than mode A, since the CISGdissipates a portion of the recovered energy to reduce the burden on theenergy storage device.

FIG. 10 is a control routine that may be performed by the control systemto achieve driveline braking via mode B. At 1010 it may be judgedwhether driveline braking is to be performed via mode B. If the answerat 1010 is no, the routine may return. Alternatively, if the answer at1010 is yes, it may then be judged whether the transmission is engagedat 1012.

If the answer at 1012 is yes (i.e. the transmission is engaged), thecontrol system can disengage the transmission at 1014 by increasing theslip in the torque converter and by transitioning the transmission intoneutral. For example, the transmission can be fully disengaged in modeB. Alternatively, if the answer at 1012 is no or from 1014 where thetransmission is disengaged, the ERAD may be controlled at 1016 toprovide the requested driveline braking force (or torque). At 1018, afirst portion of the energy generated by the ERAD may be stored at thebattery or other energy storage device. At 1020, a second portion of theenergy generated by the ERAD can be used to power the CISG, therebydissipating excess energy that cannot be absorbed by the energy storagedevice. Finally, the routine may return.

Mode C

As described with reference to FIG. 6, the control system can selectbraking mode C if the requested driveline braking force is greater thanthe braking capability of the ERAD and if the energy storage device iswithin its energy storage and energy exchange limitations. FIG. 11 isschematic illustration of the energy flows during performance ofdriveline braking mode C. Depending on the particular configuration ofthe ERAD system, it may not be able to provide the requested drivelinebraking force during all vehicle braking conditions, the transmissioncan be engaged so that the braking capabilities of the engine and CISGmay also be utilized. The requested driveline braking force can beachieved by increasing and/or maximizing the braking contributions fromthe ERAD and/or CISG while also reducing and/or minimizing the brakingcontribution from the engine. In other words, engine braking can bereduced by selecting the highest gear possible and delaying downshiftsin order to maximize energy recovery, for example, as described withreference to FIG. 15.

For a given gear ratio of the transmission, the ERAD and CISG generatingtorques in be controlled to supplement the engine braking to meet therequested driveline braking force. In order to meet the requestedbraking force, the total desired driveline braking force may first bedistributed between a desired ERAD braking force and a desiredtransmission output braking force, where the ERAD braking force iscommanded within its maximum or threshold capability, for example, asdirected by the following equations:F _(B) _(—) _(DES) =F _(B) _(—) _(ENG) +F _(B) _(—) _(CISG) +F _(B) _(—)_(ERAD) =F _(B) _(—) _(TO) _(—) _(DES) +F _(B) _(—) _(ERAD)where

$\left. \Rightarrow T_{M\_ DES} \right. = {{- \frac{F_{B\_ ERAD} \cdot R_{TIRE}}{i_{ERAD} \cdot i_{FD} \cdot \eta_{FD}}} \leq T_{M\_ MAX}}$F_(B) _(—) _(DES): desired driveline braking force, F_(B) _(—) _(TO)_(—) _(DES): desired transmission output braking force T_(M) _(—)_(MAX): max ERAD generating torque

In order to increase energy recovery and achieve the desiredtransmission output braking force, the highest gear (lowest ratio) canbe selected such that engine braking is reduced and the CISG brakingforce is increased as described by the following equation: since

$\begin{matrix}{F_{{B\_ TO}{\_ DES}} = {F_{B\_ ENG} + F_{B\_ CISG}}} \\{= F_{B\_ CISG}} \\{= {F_{B\_ DES} - F_{B\_ ERAD}}} \\{= \left. \frac{\left( {{- T_{ENG}} - T_{CISG\_ DES}} \right) \cdot i_{G} \cdot \eta_{G} \cdot i_{FD} \cdot \eta_{FD}}{R_{TIRE}}\Rightarrow \right.} \\{\min\left( {{i_{G}\text{:}F_{B\_ TO}}❘_{T_{CISG\_ DES} = 0}{< F_{{B\_ TO}{\_ DES}}}} \right.} \\\left. {{F_{{B\_ TO}{\_ DES}} \leq F_{B\_ TO}}❘_{T_{CISG\_ DES} = {\max\mspace{14mu}{capability}}}} \right)\end{matrix}$in other words select lowest gear ratio, i_(G), such that F_(B) _(—)_(TO)≧F_(B) _(—) _(TO) _(—) _(DES) while maximizing T_(CISG) _(—) _(DES)where T_(ENG): actual engine braking torque, T_(CISG) _(—) _(DES):desired CISG generating torque F_(B) _(—) _(TO) _(—) _(DES): desiredtransmission output braking force, F_(B) _(—) _(TO): actual transmissionoutput braking force, F_(B) _(—) _(ERAD): braking force from ERAD

Note that as the ERAD and/or CISG braking capabilities decrease, forexample, with increasing SOC of the energy storage device, enginebraking may be increased to achieve the requested driveline brakingforce by downshifting the transmission. Thus, as the battery SOCincreases, the driveline braking contribution can be transitionedtowards more engine braking and less braking from the CISG and ERAD.Note that the transmission can be downshifted to the lower gear toincrease engine braking as long as the maximum or threshold engine speedlimits are not violated.

In terms of energy recovery, braking mode C can provide substantialbattery charging capability. However, the amount of energy recovered maybe reduced with each downshift of the transmission since engine brakingmay be increased proportionately. Note that mode C may be used toachieve the highest braking capability of the other modes since theengine, ERAD, and CISG can be used to provide negative driveline torqueas long as the operating state of the energy storage device permitsbraking mode C operation.

FIG. 12 is a control routine that may be performed by the control systemto achieve driveline braking via mode C. At 1210 it may be judgedwhether driveline braking is to be performed in mode C. If the answer at1210 is no, the routine may return. Alternatively, if the answer at 1210is yes, it may then be judged whether the transmission is disengaged at1212.

If the answer at 1212 is yes (i.e. the transmission is disengaged), thecontrol system can engage the transmission at 1214 by reducing the slipin the torque converter or by transitioning the transmission into one ofthe torque transmitting gears. Further, upon engagement of thetransmission, the engine can provide at least a first portion of therequested driveline braking force. Alternatively, if the answer at 1212is no or from 1214 where the transmission is engaged, the transmissiongear may be varied responsive to the battery SOC while observing enginespeed limits to increase and/or maximize energy recovery. For example,the control system may seek to delay downshifts of the transmission,which would increase engine braking until the battery SOC or powerexchange limitations are approaching capacity. As another example, thecontrol system may shift the transmission to a higher gear to reduceengine braking, thereby increasing energy recovery via the ERAD and/orCISG.

At 1218, the ERAD can be controlled to provide a second portion of therequested driveline braking force in addition to the engine. As onenon-limiting example, the ERAD may be operated to provide a brakingforce that is at or near its braking threshold (at least duringconditions where battery charging is requested), whereby the remainingdriveline torque may be transmitted through the transmission where itmay be absorbed by the engine and/or CISG. In other words, thetransmission state may be controlled to transmit a level of torque thatis not absorbed by the ERAD by selecting an appropriate transmissiongear and/or by varying the torque converter state.

At 1220, the CISG can be controlled to provide a third portion of therequested driveline braking force in addition to the engine and ERAD. At1222, the energy generated by the ERAD and CISG during the brakingoperation can be stored at the energy storage device. Finally, theroutine may return.

Mode D

FIG. 13 is a schematic illustration of driveline braking control in modeD. As the SOC of the energy storage device reaches the maximum allowablelimit or energy exchange limit (e.g. power limit) and if the requesteddriveline braking force is greater than the maximum or threshold brakingforce of the ERAD, state D can be selected for driveline brakingcontrol. In order to achieve the requested driveline braking force,engine braking can be controlled by selecting the appropriate gear forthe given vehicle speed. The transmission gear can be selected such thatdriveline braking force is at least equal to or greater than therequested braking force as described by the following equation:

$F_{B\_ DES} = {F_{B\_ ENG} = \frac{\left( {- T_{ENG}} \right) \cdot i_{G} \cdot \eta_{G} \cdot i_{FD} \cdot \eta_{FD}}{R_{TIRE}}}$

-   -   select gear ratio, i_(G), such that F_(B) _(—) _(ENG)≧F_(B) _(—)        _(DES)    -   where T_(ENG): engine braking torque, F_(B) _(—) _(DES): desired        driveline braking force,    -   F_(B) _(—) _(ENG): engine braking force

If there is excessive engine braking in the lower gear beyond therequested braking force, the CISG and/or ERAD can be temporarily used tooffset the undesirable engine braking by supplying torque to thedriveline. During operation in mode D, the CISG and ERAD can becontrolled to provide zero torque so that engine braking is used as themain driveline braking control. However, it should be appreciated thatin some embodiments, the ERAD and CISG may apply a relatively smallamount of driveline braking torque compared to the engine. As such, modeD can be used where no energy recovery is desired. As one non-limitingexample, the control strategy can minimize or reduce operation in mode Dto conditions where the energy storage device is unable to acceptadditional energy.

FIG. 14 is a control routine that may be performed by the control systemto achieve driveline braking via mode D. At 1410 it may be judgedwhether driveline braking is to be performed in mode D. If the answer at1410 is no, the routine may return. Alternatively, if the answer at 1410is yes, it may then be judged whether the transmission is disengaged at1412.

If the answer at 1412 is yes (i.e. the transmission is disengaged), thecontrol system can engage the transmission at 1414 by reducing the slipin the torque converter and/or by transitioning the transmission intoone of the torque transmitting gears. Upon engagement of thetransmission, the engine can provide at least the requested drivelinebraking force. Alternatively, if the answer at 1412 is no or from 1414where the transmission is engaged, the transmission gear selection maybe varied at 1416 responsive to the vehicle speed and requesteddriveline braking force while observing engine speed limits. Asdescribed above, where engine braking exceeds the requested drivelinebraking force, the ERAD and/or the CISG can be operated to supply torqueto the driveline to offset the engine braking, thereby achieving thedesired level of braking torque.

FIG. 15 is a graph depicting an example shift strategy that can beperformed by the control system at least during operation in mode C. Thegraph of FIG. 15 illustrates an example deceleration operation wheretime is represented on the horizontal axis and vehicle speed isrepresented on the vertical axis. As indicated by 1510, vehicle speeddecreases with time due to driveline braking with or without the use offriction brakes. During the braking operation, the highest gear withinthe engine and/or transmission speed limitations may be selected whileconsidering state of the energy storage device, and braking limitationsof the ERAD and CISG. As the vehicle speed is reduced over time due todriveline braking, the transmission may be downshifted to avoidlimitations of the energy storage device, engine speed constraints,and/or braking limitations of the ERAD and/or CISG. As described abovewith reference to mode C, transmission shifts can be delayed to reduceengine braking and enable increased energy recovery via the ERAD and/orCISG as indicated at 1520.

FIG. 16 is a flow chart depicting an example control strategy forapportioning braking torque between at least two motors of thepropulsion system to provide efficient energy recapture and thermalprotection of the driveline components. Note that in this example, thefirst motor may include the CISG or the ERAD and the second motor mayinclude the other of the CISG or ERAD.

At 1610, it may be judged whether to perform driveline braking. If theanswer at 1610 is no, the routine may end. If the answer at 1610 is yes,the braking torque may be apportioned between at least the first and thesecond motors based the efficiency of each of the motors for the givenbraking torque in order to increase or maximize energy recapture. Forexample, the first motor may be operated to provide a threshold amountof braking torque to the driveline based on the efficiency of the firstmotor at converting the braking torque to energy storable by the energystorage device. As one example, efficiency may be judged based on anamount of electrical energy generated by the first motor for the levelof vehicle braking provide by the first motor. As another example, thecontrol system may utilize values or look-up tables stored in memory toidentify efficient operating ranges for the motor based on operatingconditions such as motor speed, motor torque, motor temperature, andelectrical energy generated by the motor, among other operatingconditions.

Where the braking torque provided by the first motor is less than alower torque threshold, the braking provided by the second motor and/orengine may be reduced to enable an increase in the amount of brakingtorque provided by the first motor. Alternatively, where the brakingtorque provided by the first motor is greater than an upper torquethreshold, the amount of braking torque provided by the second motorand/or engine may be increased to enable a reduction in the amount ofbraking torque provided by the first motor. As one non-limiting example,the driveline braking mode may be transitioned from one of Modes A-D toanother of Modes A-D to facilitate the apportioning of braking torqueamong the motors in order to improve the efficiency of the energyrecapture operation. In this way, the braking torque may be apportionedbetween at least a first and a second motor so that the motors areoperated to efficiently recapture energy during a braking operation.

At 1614, the thermal conditions of the first and second motors may beidentified. As one example, the motors may each include a temperaturesensor that is operatively coupled to the control system. As anotherexample, the temperature of the first and second motors may be inferred,for example, based on past and/or current operation of the motors. At1616, it may be judged whether the thermal condition of the first motoris greater than a threshold. For example, the control system may judgewhether the temperature of the first motor is greater than a temperatureset point. If the answer is no, the routine may return or may performthe operation at 1616 for the second motor. Alternatively, if the answerat 1616 is yes, the routine may proceed to 1618.

At 1618, the braking performed by the first motor may be reduced and thelevel of braking performed by the second motor may be increased inproportion to the reduction in braking by the first motor.Alternatively, the braking performed by the engine and/or second motormay be increased in proportion to the reduction in braking by the firstmotor. In this way, the temperature of the first motor may be reduced byreducing the amount of braking torque converted to energy storable bythe energy storage device.

At 1620, it may be judged whether a thermal condition of the secondmotor has reached or exceeded a threshold. For example, the approachtaken for identifying the temperature of the second motor may includeone of the approaches describe with reference to 1616 for the firstmotor. Note that the thermal threshold for the second motor may be thesame or different from the first motor. If the answer at 1620 is no, theroutine may end. Alternatively if the answer at 1620 is yes, the brakingtorque provided by the second motor may be reduced and the brakingtorque provided by the engine may be increased proportionately at 1622.Alternatively, or additionally, the amount of braking provided by thefriction brakes may be increased to supplement the reduction in brakingby the first and/or the second motors.

In this way, the total braking torque provided by the vehicle drivelinemay remain substantially the same, while adjusting the amount of brakingtorque provided by each of the motors and the engine in response tothermal conditions and the efficient operating range of the motors.Thus, the motors may be protected from thermal degradation whileproviding efficient energy recapture. As one non-limiting example, thedriveline braking mode may be transitioned from one of Modes A-D toanother of Modes A-D to redistribute braking torque among the motorsbased on thermal conditions.

In summary, the control strategies presented herein take advantage ofthe regenerative capability of the hybrid powertrain in coordinationwith transmission state selection to provide enhanced negative drivelinetorque control. Benefits of utilizing such a control strategy includereducing the frequency of transmission shifting during a brakingoperation by operating the ERAD and/or enabling the selection of asuitable braking mode from a plurality of braking modes. It should beappreciated that with each of the braking modes described herein, thecontrol system can utilize the friction brakes where appropriate tosupplement driveline braking in order to achieve the total desiredvehicle braking force.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied withvarious combinations of different engine, transmission, motorconfigurations. The subject matter of the present disclosure includesall novel and nonobvious combinations and subcombinations of the varioussystems and configurations, and other features, functions, and/orproperties disclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application. Such claims, whether broader, narrower, equal, ordifferent in scope to the original claims, also are regarded as includedwithin the subject matter of the present disclosure.

1. A method of controlling a hybrid propulsion system for a vehicleincluding at least an internal combustion engine and an electric motor,wherein said engine is coupled to at least one drive wheel of thevehicle via a transmission, the method comprising: receiving a vehiclebraking request; adjusting a state of the transmission to increase alevel of braking force provided to the drive wheel of the vehicle by theengine in response to the vehicle braking request; and varying a levelof torque exchanged between the electric motor and the drive wheelresponsive to the level of braking force provided by the engine.
 2. Themethod of claim 1, wherein the electric motor is coupled to an oppositeside of the transmission from the engine.
 3. The method of claim 1,wherein the electric motor is coupled directly to a crankshaft of theengine.
 4. The method of claim 1, wherein said varying the level oftorque exchanged between the electric motor and the drive wheel includesabsorbing torque from the drive wheel at the motor to further increasethe level of braking force.
 5. The method of claim 1, wherein said motorconverts torque absorbed from the drive wheel to electrical energy, andwherein the method further comprises storing at least a first portion ofsaid electrical energy at an energy storage device of the vehicle; andsupplying a second portion of said electrical energy to a second motorof the vehicle and operating said second motor to supply torque to saidengine.
 6. A method for a hybrid propulsion system for a vehicle, thevehicle comprising at least one drive wheel, a first motor coupled tothe drive wheel, a second motor, a transmission including a first endcoupled to the drive wheel and a second end coupled to the second motor,and a control system, the method comprising: controlling operation ofthe first motor to provide vehicle braking torque; controlling operationof the second motor to provide vehicle braking torque; and adjusting atransmission state in response to an amount of braking torque providedby the first motor and an amount of braking torque provided by thesecond motor.
 7. The method of claim 6, wherein the transmission stateis an engaged state of the transmission.
 8. The method of claim 6,wherein the transmission state is a gear of the transmission.
 9. Themethod of claim 6, wherein the transmission state is a torque ratio ofthe transmission and wherein the torque ratio is increased responsive toan increase in the amount of braking torque provided by the secondmotor.
 10. The method of claim 6, wherein the control system is furtherconfigured to perform a first mode, wherein braking torque is providedby the first motor, the second motor does not provide braking torque,and wherein the transmission is disengaged.
 11. The method of claim 10,further comprising an internal combustion engine coupled to the secondmotor, wherein the control system is configured to perform a second modewherein the first motor provides vehicle braking torque, the secondmotor provides torque to rotate the internal combustion engine of thevehicle, and wherein the transmission is disengaged.
 12. The method ofclaim 11, wherein the control system is further configured to perform athird mode, wherein at least the engine and the first motor providebraking torque and the transmission is engaged.
 13. The method of claim12, wherein the control system is further configured to perform a fourthmode, wherein at least the engine provides braking torque and the firstmotor does not provide braking torque.
 14. A method for a hybridpropulsion system for a vehicle, comprising at least one drive wheel; afirst motor coupled to the drive wheel, said first motor configured togenerate electrical energy from kinetic energy received at the drivewheel; a second motor configured to generate electrical energy fromkinetic energy received at the drive wheel; a transmission including afirst end coupled to the first motor and a second end coupled to thesecond motor; and a control system, the method comprising: varying arelative level of electrical energy generated by the first motor inresponse to an operating condition to provide vehicle braking; andvarying a relative level of electrical energy generated by the secondmotor in response to the operating condition to provide vehicle braking.15. The method of claim 14, wherein the operating condition includes atorque ratio of the transmission.
 16. The method of claim 14, whereinthe control system is configured to vary a selected gear of thetransmission responsive to a relative level of electrical energygenerated by the first motor and the second motor.
 17. The method ofclaim 14, wherein the operating condition includes a rate ofdeceleration of the drive wheel.
 18. The method of claim 17, wherein thelevel of electrical energy generated by the second motor is increasedrelative to the level of electrical energy generated by the first motorwith an increasing rate of deceleration of the drive wheel.
 19. Themethod of claim 14, wherein the hybrid propulsion system for the vehiclefurther comprises an internal combustion engine coupled to the secondmotor, and wherein the control system is further configured to adjust aspeed ratio between the first end and the second end of the transmissionin response to a rotational speed of the engine.
 20. The method of claim14, wherein the operating condition includes an efficient operatingrange of at least one of the first motor and the second motor.